Another issue that bothers practically every composer once in a while is to subconsciously plagiarize existing music. Everybody writing music knows the feeling of asking oneself the question of whether that isn’t something everybody knows already from somewhere else but you can’t point the finger on it where it is from exactly.
Statistically it is practically not avoidable to rip off existing music once in a while considering how many musical possibilities there are in tonal music combined with how many musical possibilities our ear finds pleasing in a specific style. Music lives a great deal from falling into familiar structures and our brain seems drawn to certain things that music uses over and over again so the desire by many especially young composers to be 100% original has the danger of needing to go ways that the broad audience finds rather unpleasing.
If you have the feeling that the piece you just wrote exists already, it might be advisable to ask a few other people with broad musical knowledge whether they might have heard it before. If they cannot recognize it, you should probably go ahead as sometimes it’s also just the feeling that something might exist already but actually it’s just very close to already used paths in music.